MIT chemists have devised a way to observe the transition state of the chemical reaction that occurs when vinyl cyanide is broken apart by an ultraviolet laser. Image: Christine Daniloff, MIT
New technique for observing reaction products offers insights into the chemical mechanisms that formed them. During a chemical reaction, the molecules involved in the reaction gain energy until they reach a "point of no return" known as a transition state. Until now, no one has glimpsed this state, as it lasts for only a few femtoseconds (quadrillionths of a second). However, chemists at MIT, Argonne National Laboratory, and several other institutions have now devised a technique that allows them to determine the structure of the transition state by detailed observation of the products that result from the reaction. "We're looking at the consequences of the event, which have encoded in them the actual structure of the transition state," says Robert Field, the Robert T. Haslam and Bradley Dewey Professor of Chemistry at MIT. "It's an indirect measurement, but it's among the most direct classes of measurement that have been possible." Field and his colleagues used millimeter-wave spectroscopy, which can measure the rotational-vibrational energy of reaction product molecules, to determine the structure of the products of the breakdown of vinyl cyanide caused by ultraviolet light. Using this approach, they identified two different transition states for the reaction and found evidence that additional transition states may be involved.
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